The overall goal of this research is to reveal the missing substrate X and shed light on the reaction mechanism for a phenylalanine transfer RNA (tRNAPhe) modification enzyme TYW1. TYW1 catalyzes the N1- methylguanine (m1G) modification to imG-14, a guanine derivative with a tricyclic aromatic ring. This reaction is the key step in wybutosine (base Y) biosynthetic pathway. Malfunction of TYW1 is suggested in HIV infected cells, which leads to the absence of base Y in tRNAPhe and causes -1 frame shifting to increase 400% during translation. This frame shifting is the ONLY way for HIV to produce reverse transcriptase, the key enzyme for virus replication. Thus, elucidating the mechanism of TYW1 via fundamental research is of great significance to the understanding of AIDS development and could potentially provide a novel approach in battling this deadly disease. TYW1 is found to possess a C-X3-C-X2-C motif, which is the characteristic feature of the radical SAM superfamily. The enzymes in this family utilize a unique [4Fe-4S] cluster to reductively cleave the S- adenosylmethionine, generating the 52-deoxyadenosyl (52-dA) radical. However, how this 52-dA radical catalyzes the N1-methylguanine modification in TYW1 is unclear due to the unknown nature of the second enzyme substrate X. This proposal is therefore devoted to revealing the substrate X and removing the last obstacle in mechanistic elucidation of this important enzyme. Chemical, biochemical, spectroscopic, and enzyme kinetic methods will be employed in our experimental approach. The TYW1 enzyme expressed from different resources will be utilized in this study. In addition, tRNAPhe containing either m1G or m1G analog prepared via chemical synthesis will be employed in our investigation as well. These experiments will enable us to uncover the nature of substrate X. The revealed structure of X could additionally shed light on the reaction mechanism of TYW1, which will be tested in this proposal as well as in the future investigations after the R21 grant period.